1. Field of the Invention
The present invention relates to an apparatus for the controlled cooling of hot-rolled sections, particularly beams, directly from the rolling heat.
2. Description of the Related Art
It is known in the art that the mechanical properties of materials, particularly steel, can be influenced by thermal treatments and/or the addition of alloying elements. However, the unlimited use of the known thermomechanical rolling, particularly when rolling sections, is limited by the load bearing capacity of the roll stands because these methods require high degrees of deformation at comparatively low temperatures.
The mechanical properties can be influenced by the so-called quenching and self-tempering method or QST method. In accordance with this method, the structural components, for example, rolled sections, are quenched after the finishing pass by means of water from the rolling heat. Before the core of the workpiece has cooled, cooling is interrupted and the structure in the border area is tempered by the heat which still is present in the core.
When treating steels, the material surface and the layers below the surface are cooled during the quenching process depending on the cooling duration below the martensite starting temperature which results in the formation of martensite in the border zones. This process can be influenced through the cooling time and the cooling medium; in particular, the depth of the layer in which the martensite is formed, i.e., the penetration depth, is adjusted through the cooling time.
After the end of this forced cooling, the tempering process is carried out in which the previously formed martensite layer is tempered by the residual heat in the section. The temperature once again rises above the martensite starting temperature. This causes the stresses in the martensitic area to be relieved and, thus, a material having high strength with simultaneously good toughness is adjusted.
During the subsequent cooling process in air, bainitic and/or (finely) pearlitic structure is formed in the interior of the cross-section.
If the material is to be cooled without the formation of martensite, the surface of the sections is cooled by adjusting the cooling time and cooling intensity in such a way that the temperature does not drop below the martensite starting temperature. Also in this case, for homogenizing the temperature distribution, tempering is carried out after the end of the forced cooling. After the conclusion of the tempering process, an improvement of the mechanical properties is obtained by the adjustment of, for example, a finely pearlitic and ferritic structure.
It is important for the uniform adjustment of the desired properties that the cooling medium is applied in a specific or targeted manner to the section or the surfaces to be cooled.
An apparatus for cooling beams is disclosed in European Patent 0 140 026. The application of cooling liquid is effected by passing the beams through so-called cooling boxes. These cooling boxes are provided with openings arranged at equal distances for spraying the cooling liquid. For cooling the external and internal areas of the beam, the boxes for the outer surface have at least such a size as it corresponds to the height of the flanges. For cooling the inner flanges, the boxes extend over the entire inner surfaces of the flanges and at least 70% of the web surface. It is possible to influence the bending behavior even in the case of asymmetrical sections.
It has been found to be disadvantageous in this apparatus that an adjustment of the inner cooling boxes is required in the case of different geometries of the sections when the rolling program is changed. In addition, this method can be used predominantly in large beams because a tool is moved from the top and below between the flanges. In the case of small beams, this is not possible or only possible to a very limited extent because of the small available space.
Since the application of cooling water to the beam takes place for all locations of a surface approximately equally and because of the configuration of the cooling boxes individual openings or rows cannot be switched off, it is necessary to have a temperature distribution over the cross-section which is as homogeneous as possible. Consequently, this temperature distribution must be adjusted to a certain extent already during the rolling process by means of a selective cooling of the transition zone between flange and web. In addition, this apparatus is not capable of carrying out cooling so as to reduce the internal stresses of the beam.
European Patent 0 462 783 discloses a method and an apparatus for the thermal treatment of thin-walled I-sections. In this method, a forced cooling of the rolled products takes place between the rolling procedures. The cooling apparatus itself is composed of a plurality of nozzles which are arranged one on top of the other. They are operated with water and can be differently switched on and off. However, the described cooling apparatus only cools the outer sides of the flanges of the sections. This is done with the purpose of cooling the outer surface of the beam prior to the subsequent hot rolling to a temperature of 700.degree. C. or less. By repeating the forced cooling with water, the microstructure of the flange surface is transformed up to a certain depth.
A cooling method and apparatus is also disclosed in European Patent 0 098 492. In that case, an apparatus is proposed for cooling steel sections, specifically rails, which are guided through a cooling apparatus. A variation of the cooling process or local cooling are achieved by different orientations of the rails being transported as well as through baffle plates for the cooling medium.